Field of the Invention
The present invention relates to an image processing technique for an object image.
Description of the Related Art
Measuring methods using a phase as means for precisely measuring substances have been conventionally used. Measuring methods using a phase involve causing interference with (coherent) incident light that has aligned wave fronts and measuring resultant interference fringes to determine a change in incident light wave front (phase) due to a phase difference equal to one severalth to one several-tenth of the wavelength. An interferometer using such a measuring method is suitable means for measuring, for example, slight concaves and convexes on a surface of a lens.
Among the wave front measuring methods using interference, attention has recently been paid to an X-ray phase imaging utilizing light (electromagnetic waves) with a wavelength of several tens of nanometers or shorter, that is, X rays. Unlike X-ray absorption imaging in which contrast resulting from absorption by an object is imaged, the X-ray phase imaging involves detecting, based on phase interference, a change in the length of an optical path of incident light formed during transmission of X rays through the object.
As an example of the X-ray phase imaging, Talbot interferometry using X rays will be described. In an X-ray Talbot interferometer, X rays from a light source are transmitted through an object to change an incident phase of light. Light transmitted through the object is diffracted by a grating referred to as a diffraction grating and having a periodic pattern, to form a first interference pattern at a position located at a predetermined distance referred to as a Talbot length, from the diffraction grating. A change in the first interference pattern in this case is analyzed through a comparison with a change in the first interference pattern in a case where the object is not present, to determine a change in incident light wave front.
The pattern period of the diffraction grating with the periodic pattern as described above varies according to conditions such as the length of the apparatus and the wavelength of the incident light. Typical X rays have a period of the order of several micrometers. As is known, the first interference pattern formed by the X rays also has a period of the order of several micrometers. In such a case, common detectors have a resolution of at most several tens of micrometers and thus fail to detect the first interference pattern. Thus, a shield grating having substantially the same period as that of the first interference pattern is arranged at a position where the interference pattern is formed. The shield grating blocks a portion of the first interference pattern to form a second interference pattern with a period of approximately several hundred micrometers, that is, a moiré pattern. Then, the moiré pattern is detected by a detector to allow a change in the interference pattern to be indirectly measured. Examples of a method for forming a moiré pattern include a method of aligning the direction of the shield grating with the period adjusted with the direction of the first interference pattern formed by the first grating (enlarged moiré pattern) and a method of rotating the grating to adjust the period and direction of the moiré pattern (rotated moiré pattern).
Numerical analysis of the moiré pattern allows several parameters relating to the object to be acquired. In a typical example of parameter acquisition, a difference in the length of the optical path of the incident light is detected as a differential phase. This allows the refractive index for the object to be calculated. Such images based on the refractive index advantageously exhibit a higher signal-to-noise ratio (SN ratio) than X-ray absorption images particularly for some types of objects such as soft tissues and plastics.
In recent years, studies have been conducted on techniques for acquiring information on a small-angle scattering of not more than the order of pixels for the object by calculating a change in the amplitude of the moiré pattern. In other words, a two-dimensional X-ray Talbot interferometer allows, in addition to the conventional absorption image, images representing independent physical quantities such as a differential phase image, an amplitude image, and a scattering image to be acquired during one imaging process.
Moreover, in recent years, two-dimensional X-ray Talbot interferometers including a two-dimensional grating have been actively studied. The two-dimensional X-ray Talbot interferometers are characterized by, for example, being capable of simultaneously acquiring a differential image in an X axis direction and a differential image in a Y axis direction that is perpendicular to the X axis. The two-dimensional X-ray Talbot interferometers can thus acquire more accurate refractive-index information. In US Patent Application Publication No. US 2014/0153692, a method is disclosed in which a clear image of the object is acquired from such differential phase images in the two directions as described above. The method in US Patent Application Publication No. US 2014/0153692 enables an edge boundary of the object to be clarified utilizing inverse Riesz transform and is expected to further enhance the SN ratio for the contour of the object. Alternatively, the Laplacian may be determined by further differentiating differential phases.
Such techniques are very effective for modalities such as X-ray interferometers which use X rays, particularly when the object is susceptible to exposure such as a living organism. For such objects, an X-ray exposure dose is desirably as low as possible. Naturally, a trade-off exists between a reduction in exposure dose and a decrease in SN ratio, and a technique is desired which emphasizes the edge more significantly for the final image.
A technique for effectively emphasizing the edge is disclosed in, for example, International Publication No. WO 2010/034968. The technique in International Publication No. WO 2010/034968 is an improved technique for noise reduction that enables emphasis only of an information portion desired to be effectively emphasized by repeating a process of applying an anisotropic noise reduction filter in accordance with the location and shape of the object. Such a technique can be applied to the X-ray Talbot interferometer. However, the technique in International Publication No. WO 2010/034968 needs to pre-achieve a given SN ratio for an image obtained as primary information so that the direction of the anisotropic filter can be detected. On the other hand, when the exposure is reduced as much as possible for the purpose of less exposure, the SN ratio may decrease to such a degree that the technique in International Publication No. WO 2010/034968 is difficult to apply (a degree that the direction of the anisotropic filter cannot be detected).
One of such examples is imaging of a cartilage in the living organism. The cartilage cannot be imaged by conventional absorbed X rays and is one of imaging targets to which the X-ray Talbot interferometer is expected to be applied. The cartilage itself exhibits a high refractive index in vacuum. However, the cartilage present in a living tissue exhibits a lower relative refractive index between this living tissue and another living tissue, preventing a sufficient contrast from being achieved, though imaging is possible.